1. Field of the Invention
The present invention generally relates to a multiple antenna system that can operate in either multiple input multiple output (MIMO) mode, a beam forming/steering mode or a diversity mode.
2. Description of the Related Art
Many communication systems, and in particular, wireless communication systems use antennas to convey (i.e., transmit and receive) communication signals. In many cases, a plurality of antennas is used to convey the signals. The plurality of antennas, commonly referred to as an antenna array, is often used to increase the amount of information (e.g., information transfer rate or throughput) that is being conveyed or to improve the quality of a signal being transmitted (or received) by the antenna array. One technique used with antenna arrays to improve the quality of a signal being transmitted (or received) is called spatial diversity.
Spatial diversity is the selection of a particular antenna or a group of antennas from an array of antennas to transmit (or receive) a communication signal. A signal transmitted with a spatially diverse structure will have the signal taking different paths to its ultimate destination. An example of a transmit diversity scheme is space time spreading (STS) which is used as part of the standard for IS-2000 wireless communication networks. To further improve the quality of a transmitted spatially diverse signal, the transmitted diverse signal can also be processed by a beam forming/steering device; beam forming/steering in combination with STS is called Steered STS. In beam forming/steering or Steered STS the antenna is coupled to a device which controls the relative phase of the signal being transmitted by each antenna in order to form a focused beam in a particular direction in space. The beam can control gains (transmit or receive) for specific users or can control gains based on the direction of the signal. Also the gain can be related to only the signal strength or both the signal strength and interference strength thus maximizing the Carrier to Interference ratio (C/I). The Steered STS technique can be used in many wireless communication networks. A similar scheme called Space Time Transmit Diversity (STTD) has been defined for Universal Mobile Telecommunications System (UMTS) networks. The steered STS approach can be used with any diversity schemes such as STS or STTD.
Another technique used with antenna arrays is called multiple input multiple output (MIMO). Unlike spatial diversity techniques wherein a group of antennas is used to transmit (or receive) a single signal, MIMO techniques use an antenna array coupled to a signal processing device (including transmission and reception circuitry) to simultaneously transmit and/or receive multiple distinct signals. A particular example of a MIMO system is the BLAST (Bell Labs LAyered Space Time) scheme conceived by Lucent Technologies headquartered in Murray Hill, N.J. In BLAST each transmit antenna is used to either transmit or receive distinct signals. Various coded BLAST schemes exist (e.g., diagonal BLAST or D-BLAST; vertical BLAST or V-BLAST) where each signal is coded prior to being transmitted. In a BLAST device that uses coding, often the same code is used for each of the distinct signals; this is called code reuse.
Antenna arrays are typically intended to operate in multipath environments in which communication signals transmitted by an antenna do not propagate in a straight line towards a receive antenna. Rather, in a multipath environment, the communication signals scatter off various objects (e.g., buildings, trees) located between a transmit antenna and a receive antenna. Thus, a multipath environment creates a multitude of possible paths for a signal going from a transmit antenna to a receive antenna. The BLAST technique exploits a multipath environment by using multiple transmitters and receivers to create, in effect, a plurality of independent subchannels each carrying independent information. The communication signals occupy the same bandwidth simultaneously and thus spectral efficiency increases with the number of independent subchannels. Theoretically, the more scattering that occurs in the multipath environment, the more subchannels that can be supported. Therefore, antenna arrays of a communication system that use the BLAST technique serve to increase the information throughput of a communication network.
For certain situations and for certain types of communication signals, it is desirable to use one technique over another. For example, for voice signals where the quality of the voice signal being transmitted and the capacity (i.e., number of voice users supported) are crucially important, it is typically advantageous to use beam forming/steering. In other situations in which data signals (i.e., text, graphics, Internet data) are being transmitted, the issue of information throughput is often paramount compared with other factors thus making usage of MIMO techniques desirable in such circumstances. The configuration of antenna arrays for the two techniques (beam forming/steering and MIMO) have contradicting spatial requirements. In particular, antenna arrays that perform beam forming/steering exploit the correlation aspects of the signals being transmitted by antenna elements proximately positioned with respect to each other.
Signal correlation is a phenomenon whereby the variations in the parameters (i.e., amplitude and phase) of a first signal of a first antenna track the variations in the parameters of a second signal of a second antenna in the vicinity of the first antenna. In general, as the spacing between antennas increases, the correlation between signals being transmitted (or received) by the antennas decreases. Conversely, as the spacing between antennas decreases, the correlation between signals being transmitted (or received) by the antennas increases. To achieve relatively highly correlated signals in typical wireless communication systems, the spacing between antennas is of the order of       1    2    ⁢  λ
or less where xcex is equal to   c  f
which is the wavelength corresponding to the largest frequency (ƒ) within a band of frequencies at which the antennas are operating; c is the well known constant representing the speed of light in vacuum. It is desirable to have relatively high correlation between signals transmitted (or received) by antennas being used for beam forming/steering applications such as Steered STS. On the other hand, it is desirable to have a relatively low correlation or no correlation between antennas when they are used for MIMO applications such as BLAST or diversity applications.
Communication systems may use antenna arrays configured to perform beam forming/steering through the use of steered STS. In many situations, these same communication systems have a need to have BLAST capability or diversity capability. In order to perform BLAST operations with their current antenna configurations, such communication systems have to deploy additional antennas appropriately spaced with respect to each other and to the existing antennas. Not only is the deployment of additional antennas a cost increase for service providers, it also presents an environmental and esthetic concern for many communities within which communication towers comprising base station equipment and antennas are located. Service providers are entities that own, operate and control communication networks and their associated equipment. What is therefore needed is an antenna array in which beam forming/steering, MIMO and diversity operations can be performed on signals being transmitted and/or received without having to deploy additional antennas. What is further needed is an antenna array configuration that can perform either MIMO or beam forming/steering or diversity operations, or an antenna array, which simultaneously performs beam forming/steering, MIMO, diversity operations or any combination thereof.
The present invention is an antenna array comprising at least two antenna groups where each group comprises at least two pairs of antennas where each pair selectively operates in either a MIMO mode, a beam forming/steering mode, a diversity mode or any combination thereof. Each pair of antennas within a group is orthogonally polarized and each antenna in a pair is selectively activatable. One antenna from at least one of the pairs in the group is similarly polarized with one antenna from at least one other pair in the group. A first group of antennas is positioned with respect to a second group of antennas such that there is relatively low correlation or no correlation between signals being transmitted (or received) by any one of the antennas from different groups. The groups are configured such that they operate either in a MIMO mode, a beam forming/steering mode, a diversity or any combination thereof. The groups in the antenna array are coupled to circuitry that cause certain antennas in a group to be selected and activated so that each group is able to operate in any one of the aforementioned modes.
The antenna groups are coupled to the circuitry via switches which are activated by control signals from the circuitry or are designed to automatically route signals to certain antennas based on certain characteristics of the signals to be transmitted and/or being received. The switches are designed such that they are able to determine certain characteristics from a signal and route that signal to a proper antenna such that the corresponding group to which the switch is coupled can operated in either the MIMO, beam forming/steering or diversity modes. In a preferred embodiment of the present invention, the circuitry is able to determine certain characteristics of signals to be transmitted or being received by the antenna array and, based on the determined characteristics, generate control signals which activate the proper switches that activate certain antennas in a group to cause the group to operate in any one of the three modes or any combination of the three modes. In this manner, different groups can operate in the same mode or in different modes as determined by the circuitry.